The invention relates to cadmium-free optical steep edge filters and a method for producing same.
Optical steep edge filters are characterized by their typical transmission properties. In the short-wave range, the transmission is low. It increases to high transmission over a narrow spectral range and remains high in the long-wave range. The range of low transmission is called stop band, the range of high transmission is called pass band.
Optical steep edge filters are characterized by the following parameters:                The edge wavelength λc is the wavelength at which the spectral internal transmittance between stop band and pass band amounts to half of the maximum value.        Stop band: In the stop band, the internal transmittance factor τis does not exceed 10−5. The stop band usually begins between approx. 40 and 60 nm below λc.        Pass band: In the pass band, the internal transmittance factor τip does not fall below 0.99. The pass band begins between approx. 40 and 60 nm above λc.        
A typical example of a state of the art steep edge filter is Schott Glas OG 590, where λc is 590 nm. The stop band is reached below 540 nm (τis, 540 nm=10−5) and the pass band is reached above 640 nm (τip, 640 nm=0.99).
Deviating from the above statements, the gaps for τis and τip from λc for filter glass having an edge wavelength in the near infrared can be greater than 40-60 nm.
Optical steep edge filters according to prior art comprise glass on cadmium sulfide, cadmium selenide and/or cadmium telluride bases, such as the glass specified in DE-OS 20 26 485 and DE-PS 26 21 741. The color effect allowing it to function as a filter with steep absorption edges in the spectral range between 455 and 850 nm is based on semiconductor particles, for example micro-crystalline (Zn, Cd) (S, Se, Te) with a particle size of 1-10 nm in a glass matrix.
The disadvantage of the earlier steep edge filters is that they contain toxic and carcinogenic Cd compounds, which requires complex safety and protective measures for batch preparation, reprocessing and disposal, for example.
Another disadvantage is that the edge wavelength can be varied only within the above mentioned spectral range in the visible range and the near infrared range. Steep edge filters in the longer-wave infrared range are desired especially for filters for infrared lights, substrates for infrared band-pass filters and filters for covering infrared LEDs.
A band-pass filter for a Raman spectrometer comprising a I-III-VI2 single crystal is known from JP 03-277928.
The most important difference between a band-pass filter and a steep edge filter, as claimed in the present invention, is the fundamental principle of physics, and therefore the absorption and passing regions.
Band-pass filters can be produced by ion doping or interference coatings, for example, whose position and half-width depend on the ion type, the ion concentration or the type, the number and the thickness of the coatings. The electron transitions responsible for the absorption bands can be explained by the ligand field theory. Accordingly, electron transitions occur on individual wavelengths that correspond precisely to the energy required for transporting the electron from the energetically lowered orbitals to the energetically raised orbitals. The result is absorption at exactly said wavelength and thus a band is formed in the transmission spectrum.
In an edge filter, electron transitions take place between a conductance band and a valence band. The energy of the light must be greater than the band gap between the valence band and the conductance band so as to generate electron transitions. Light with energies of less than the band gap will not lead to electron transitions. Therefore, instead of selective electron transitions at certain wavelengths, electron transitions and thus absorption will occur up to a certain wavelength just corresponding to the energy of the band gap. Said certain wavelength represents the edge of the filter.
The aim of the invention is to provide cadmium-free optical steep edge filters with good cut-off in the short-wave range and high transmission in the long-wave range whose edge is variable over a wide wavelength range and which are environmentally compatible and easy to manufacture. In particular, the disadvantages encountered with optical steep edge filters according to prior art should be prevented.